For communication between devices, for example in automotive applications, various protocols are used. One protocol frequently employed is the SENT protocol (single edge nibble transmission). This protocol may for example be used in applications where high resolution data is transmitted for example from a sensor device to an electronic control unit (ECU).
The SPC protocol (short PWM code; PWM meaning pulse width modulation) is an extension of the SENT protocol and aims at increasing performance of a communication link and reducing system costs at the same time. To some extent, SPC allows bidirectional communication and is an example of an edge based PWM protocol. For example, SPC may introduce a half-duplex synchronous communication. A receiver (e.g. master) generates for example a master trigger pulse on a communication line by pulling it low for a defined amount of time. The pulse width (corresponding to the defined amount of time) is measured by a transmitter (e.g. slave), for example a sensor, and a transmission, e.g. a SENT transmission, is initiated only if the pulse width is within a defined limit. The SPC protocol allows choosing between various protocol modes. For example, a synchronous mode, a synchronous mode with range selection or a synchronous transmission with ID selection, where up to four sensors may be connected in parallel to an ECU, may be used. In the latter case, the pulse width of the above-mentioned trigger pulse may define which sensor or other entity will start a transmission. For example, a length of the trigger pulse may indicate an ID of a sensor or other slave device selected for transmission. The sensor or other entity may start the transmission with its own synchronization, which may overlap data pulses, e.g. with a sync pulse which may, but need not, overlap a trigger pulse, followed by data pulses.
In conventional SPC-based communication, on master side and slave side open drain outputs or current sinks are used. In a passive state none of the communication devices (master or slaves) actively drives the line, and the line is held e.g. by a pull-up resistor or, in case of current sinks, e.g. by a sensor. In the latter case, a third connection between devices may be omitted.
In such cases, a rising edge of signals may be influenced by parasitic properties of a communication line, for example by a parasitic capacitive load. This in some circumstances may lead to problems in particular in a pulse identification of the SPC protocol or other edge-based protocol as e.g. it may lead to uncertainties of a time measuring of a trigger pulse which in turn serves as an identification pulse on a slave side. Furthermore, the transmission may be disturbed by such a behavior, which may be referred to as a “higher ohmic” behavior. Disturbances by the parasitic loads may even influence the data transmission itself in some circumstances when they lead to an incorrect recognition of falling edges. A current-based transmission may overcome some of the limitations discussed above for an open-drain setup, but may cause other difficulties like higher power consumption for drivers and EMC (electromagnetic compliance) issues like robustness and emissions during a switching, as in such cases it is desirable to keep the power-dissipating current pulses as short as possible (just as long as required for reliable detection, similar to the edge detection in the open-drain mode).